"Based on ISI instruction through (former ISI chief) General Hamid Gul, Mullah Mohammad Omar was told to move from Quetta city of Pakistan to northern Waziristan," a source said.

"As Mullah Omar was being transported from Quetta to North Waziristan by the ISI, he was secretly killed by the ISI."

"This is pure propaganda. This is not possible at all," Taliban spokesman Zabihullah Mujahid said from an undisclosed location. He added that Omar was currently inside Afghanistan "leading the mujahideen".

Meanwhile, an Afghan intelligence source said Omar had been missing for 11 days following a meeting with Gul.

"It has been 11 days since his close circle, the people around him, have no clue of his whereabouts. The Taliban leadership are all concerned and worried for the sudden disappearance of Mullah Omar," the source said.

Afghanistan and Pakistan regularly trade accusations over Taliban insurgencies plaguing both their countries. There is deep distrust between Afghan intelligence and the ISI, which helped create the Taliban in the 1990s.

Omar was considered the spiritual leader of the Taliban movement that operates in parts of Afghanistan and Pakistan. He was Afghanistan`s de facto head of state from 1996 to late 2001 and had the title `Head of the Supreme Council`.